Development of a high harmonic gyrotron with an axis-encircling electron beam and a permanent magnet

A gyrotron with an axis-encircling electron beam is useful for high frequency operation, because the high beam efficiency is kept even at high harmonic of electron-cyclotron resonance. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already succeeded in operation at the third harmonic and the fourth harmonic resonances. The operation frequencies are 89.3 and 112.7 GHz, respectively. Operation cavity modes are TE₃₁₁ and TE₄₁₁. The permanent magnet system consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in cavity and electron gun regions. The magnetic field at the cavity region can be varied from 0.97 to 1.18 T. At the optimum condition of the magnetic field intensity, the output power at the third harmonic operation is 2.5 kW. The operation is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy and current are 40 kV and 1.2 or 1.3 A. Starting current, beam efficiency and emission pattern also have been measured. In this paper, the operation results of the gyrotron and comparison with the computer simulation results are described.